1. Field of the Invention
This invention relates to an improved process for the plasma etching of polysilicon substrates involving the use of distinct plasma species. A plasma can be generated when a gas, for example, chlorine, is exposed to a current of electricity under certain conditions. Although the chemical nature of plasmas are not clearly understood, it is known that a highly excited species can be made by exposing certain gases to high energy radio frequencies. Under the influence of these radio frequencies, the gases breakdown and rearrange to form transitory species with life spans so short that they are difficult to identify. Accordingly, unexpected reactions can be effected in a plasma that are difficult or impossible to effect using more conventional techniques. For example, a plasma of a very inert gas such as a fluorocarbon, known commercially as Freon, will etch glass, indicating that an active fluorine species is present in the plasma. In addition to the active chemical species, there are strong radiations, such as ultraviolet, and strong ion and electron bombardment of the surfaces within the plasma.
Considering the enigmatic properties of plasmas, certain unique problems occur in the use of specific etching species. Previous etch gases used to generate etching plasmas, such as sulfur hexafluoride, carbon tetrachloride and others, have been used by themselves, or in combination with other etching gases or with inert carrier gases such as helium, oxygen, etc. Each of these previous gases have some advantage over their counterparts, but suffer, usually, from other drawbacks. Whereas one may give very good uniformity of etching, there may be excessive undercutting (isentropicity). Another gas may have demonstrated improved selectivity in etching between the polysilicon and underlying silicon dioxide but with a decrease in etch rate.
2. Brief Statement of the Prior Art
Yamamoto, et al., U.S. Pat. No. 4,094,722 teaches a plasma etching apparatus using tetrafluoromethane (CF.sub.4) and oxygen as a carrier to etch polysilicon wafers. Apparently, the inventors did not think that any other etch gas would be feasible for the etching of polysilicon. For example, ordinarily polysilicon would not be etched by chlorine in a plasma mode. On the other hand, polysilicon can be etched by chlorine in a sputtering mode. Other inventions have shown that aluminum is easily etched by highly reactive species. That was the subject matter of applicant's co-pending application Ser. No. 928,597 filed July 27, 1978, U.S. Pat. No. 4,182,646. However, the combination of a fluorinated and a chlorinated gas has not been heretofore seen in the art.
Currently, problems in plasma etching of polysilicon reside in three areas. One area is selectivity in the amount of polysilicon layer etched in comparison with underlying silicon dioxide--naturally, the higher selectivity for polysilicon, the better. As mentioned, tetrafluoromethane has been used in conjunction with oxygen to provide an etching plasma. Selectivity of etching using tetrafluoromethane is about 5-10 to 1. Unfortunately, use of this etchant gas has been observed to etch preferentially about the outside edge of the substrate, the so-called "bulls-eye effect". Sulfur hexafluoride on the other hand, has a even greater selectivity of 10-20 to 1.
A grave problem in the use of both sulfur hexafluoride and tetrafluoromethane etchants is the phenomenom of undercutting. Undercutting occurs when the etchant gas removes polysilicon underneath the photoresist.
The last problem in polysilicon etching is in the uniformity of etching. It has been found, through experimentation, that a fluorinated gas, eg., sulfur hexafluoride, especially in combination with carbon monoxide, gives extremely good selectivity, but very poor uniformity.